This invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device with a super twisted liquid crystal material.
Liquid crystal display devices and methods of driving the devices are well known in the art. The driving methods are classified into two categories. There is the static drive method and a time sharing method. In the static drive method, a signal voltage is continuously applied to the electrodes of a display device during display on the basis of "one pixel at a time". The static drive method is less than satisfactory, however, since a large number of driving elements and lead terminals are required.
In the time-sharing addressing method, the signal voltages for display are applied to the signal electrodes on a time-sharing basis for each scanning electrode so as to provide a "one line at a time" display. This driving method is widely used for it requires fewer driving elements and lead terminals than those in the static drive method. In the time-sharing addressing method, a duty ratio is expressed in general by the term 1/N. In general, N will be a large number in excess of 200 in order to satisfy the need to display large contents in the device.
In order to drive a liquid crystal display device in the time-sharing method at a duty ratio of 1/N, it has been considered most suitable to select a bias ratio of 1/(N+1). However, this bias ratio has been selected only because it provides the maximum contrast in the display device. The selection of the bias ratio does not take into consideration or depend on the voltage of the driving signals.
If the duty ratio is lowered, the voltage of the driving signal applied must be increased. On the other hand, the integrated circiuts (IC) cannot withstand a driving voltge in excess of 2.sqroot.N.multidot.V. Thus, since the voltage applied is limited to the maximum for an integrated circuit, the device must be driven at a suitable bias ratio and the threshold voltage of the display must be lowered.
In a super twisted liquid crystal display, the display contrast quality degrades significantly with a decrease in threshold voltage of the cell. The use of the most suitable bias ratio of 1/(.sqroot.N+1) causes the display contrast property to deteriorate. Additionally, there is a delay in response time of the display due to use of the conventional bias ratio.
Accordingly, it is desirable to provide a liquid crystal display utilizing a super twisted liquid crystal which has improved display contrast quality and response time when driven by a time-share addressing method which overcomes these problems associated with the prior art.